Optical sheet, surface light source and display device

1. An optical sheet, comprising:

a translucent substrate having first and second surfaces;

an optical element provided on the first surface of the translucent substrate, the optical element including a plurality of unit prisms or unit lenses arranged thereon; and

at least one light diffusion layer provided in at least one of the following locations:

between the first surface of the translucent substrate and the optical element; and

on the second surface of the translucent substrate,

wherein the at least one light diffusion layer includes micro particles and gap parts which are formed around the micro particles, respectively, and each of the gap parts has an oval shape oriented in a uniaxial direction, and

an image contrast value of the light diffusion layer having the gap parts is equal to or less than 15 at a slit pitch of 0.5 mm, the image contrast value being measured by a method standardized by JIS K 7374.

2. An optical sheet, comprising:

a translucent substrate in which micro particles and gap parts are formed near at least one of first and second surfaces of the translucent substrate, or within an area from the first surface to the second surface of the translucent substrate, the gap parts being formed around the micro particles, respectively, and each of the gap parts has an oval shape oriented in a uniaxial direction; and

an optical element that is configured with a plurality of unit prisms or unit lenses provided on one of the first and second surfaces of the translucent substrate,

wherein an image contrast value of the translucent substrate having the gap parts is equal to or less than 15 at a slit pitch of 0.5 mm, the image contrast value being measured by a method standardized by JIS K 7374.

3. The optical sheet of claim 1 , wherein

the light diffusion layer having the gap parts is a uniaxially or biaxially stretched transparent resin layer in which micro particles are dispersed, and

the gap parts are in a flat shape in cross sectional view that extends in a direction orthogonal to a normal line of the optical sheet.

4. The optical sheet of claim 1 , wherein the gap parts are approximately oval and are oriented in the uniaxial direction in plan view.

5. The optical sheet of claim 1 , wherein the gap parts are approximately circular in plan view.

6. The optical sheet of claim 1 , wherein the light diffusion layer having the gap parts has anisotropic diffusion.

7. The optical sheet of claim 6 , wherein, when representing diffusion of light for the anisotropic optical sheet by a light diffusion curve represented by brightness and a diffusion angle, a direction in which the virtual line indicating a maximum of half bandwidths obtained from the light diffusion curve extends, is a direction orthogonal to a major axis direction of the gap parts formed in an approximately oval shape oriented in the uniaxial direction in plan view.

8. The optical sheet of claim 6 , wherein, when representing diffusion of light for the optical sheet having anisotropic diffusion by a light diffusion curve represented by brightness and a diffusion angle, a first direction in which the virtual line indicating the maximum of half bandwidths obtained from the light diffusion curve extends, and a second direction of a ridge line of the unit prisms or unit lenses structuring the optical element, are orthogonal or approximately orthogonal to each other.

9. The optical sheet of claim 6 , wherein, when representing diffusion of light for the anisotropic optical sheet by a light diffusion curve represented by brightness and a diffusion angle, a first direction in which the virtual line indicating a maximum of half bandwidths obtained from the light diffusion curve extends, and a second direction of a ridge line of the unit prisms or unit lenses structuring the optical element, are parallel or approximately parallel to each other.

10. The optical sheet of claim 1 , wherein the light diffusion layer having the gap parts has isotropic diffusion.

11. The optical sheet of claim 1 , wherein

a Hayes value of the light diffusion layer having the gap parts is equal to or greater than 20% and equal to or less than 95%, the Hayes value measured by a method standardized by JIS K 7361-1.

12. A surface light source device, comprising:

a light guide body that includes a light incidence surface and a light discharge surface, that is formed of a translucent material, and that discharges light introduced from the light incidence surface;

a light source that illuminates light to the light incidence surface and through the light guide body; and

an optical sheet that transmits the light exiting from the light discharge surface, the optical sheet including:

a translucent substrate having first and second surfaces,

an optical element provided on at least one of the first and second surfaces of the translucent substrate, the optical element including a plurality of unit prisms or unit lenses arranged thereon, and

at least one light diffusion layer provided in at least one of the following locations: between the first surface of the translucent substrate and the optical element; and on the second surface of the translucent substrate,

wherein the at least one light diffusion layer includes micro particles and gap parts which are formed around the micro particles, respectively and each of the gap parts has an oval shape oriented in he uniaxial direction, and

wherein an image contrast value of the at least one light fusion layer having the gap parts is equal to or less than 15 at a slit pitch of 0.5 mm, the image contrast value being measured by a method standardized by JIS K 7374.

13. A surface light source device, comprising: an optical sheet that comprises: a translucent substrate having first and second surfaces, an optical element provided on at least one of the first and second surfaces of the translucent substrate, the optical element including a plurality of unit prisms or unit lenses arranged thereon, and at least one light diffusion layer provided in at least one of the following locations: between the first surface of the translucent substrate and the optical element; and on the second surface of the translucent substrate, wherein the at least one light diffusion layer includes micro particles and gap parts which are formed around the micro particles, respectively and each of the gap parts has an oval shape oriented in the uniaxial direction; a light source that illuminates light from a rear side of the optical sheet; and a reflection body positioned on a side opposite from the optical sheet of the light source and reflecting the light from the light source to the optical sheet, wherein an image contrast value of the at least one light diffusion layer having the gap parts is equal to or less than 15 at a slit pitch of 0.5 mm, the image contrast value being measured by a method standardized by JIS K 7374.

14. The surface light source device of claim 12 , wherein the optical element is provided on a side of the optical sheet from which the light from the light source exits.

15. The surface light source device of claim 12 , wherein the optical element is provided on a side of the optical sheet from which the light from the light source enters.

16. A surface light source device, comprising:

a light guide body that includes a light incidence surface and a light discharge surface, is formed of a translucent material, and discharges light introduced from the light incidence surface;

a light source that illuminates light to the light incidence surface and through the light guide body; and

an optical sheet that transmits the light exiting from the light discharge surface, the optical sheet including:

a translucent substrate having first and second surfaces, including gap parts formed near the first surface of the translucent substrate or from the first surface to the second surface of the translucent substrate; and

an optical element that is provided on the first surface of the translucent substrate, the optical element including a plurality of unit prisms or unit lenses arranged thereon, and

at least one light diffusion layer provided in at least one of the following locations:

between the first surface of the translucent substrate and the optical element; and on the second surface of the translucent substrate,

wherein the at least one light diffusion layer includes micro particles and, gap parts which are formed around the micro particles, respectively and each of the gap parts has an oval shape oriented in the uniaxial direction, and

an image contrast value of the at least one light diffusion layer having the gap parts is equal to or less than 15 at a slit pitch of 0.5 mm, the image contrast value being cured by a method standardized by JIS K 7374.

17. A display device, comprising:

a flat shaped translucent display body; and

the surface light source device of claim 12 that is positioned on a rear side of the translucent display body and that irradiates light from the rear side of the translucent display body.

18. The optical sheet of claim 1 , wherein

the oval shape has a short axis and a long axis, and

the short axis has a length from 1.2 times to 4.0 times of an average particle diameter of the micro particle, and

the long axis has a length from 4.0 times to 8.0 times of the average particle diameter of the micro particle.

19. The optical sheet of claim 1 , wherein

at least one of the number, shape, size and volume of the gap parts is defined so that an image contrast value of the light diffusion layer having the gap parts is equal to or less than 15 at a slit pitch of 0.5 mm, the image contrast value measured by a method standardized by JIS K 7374.